As mobile device technology continues to develop and demand therefore continues to increase, demand for secondary batteries as energy sources is rapidly increasing. In addition, secondary batteries have recently been used as power sources for electric vehicles (EVs), hybrid electric vehicles (HEVs), and the like. Accordingly, research into secondary batteries that can meet a variety of demands is underway and, in particular, demand for lithium secondary batteries having high energy density, high discharge voltage and high output stability is increasing.
In particular, lithium secondary batteries used as power sources of EVs and HEVs require high-output characteristics that exhibit high output within a short period of time.
Conventionally, a lithium cobalt composite oxide having a layered structure is generally used as a cathode active material of a lithium secondary battery. When a lithium cobalt composite oxide is used as a cathode active material, however, cobalt as a main component is very expensive and output characteristics thereof are bad. Thus, lithium secondary batteries including such cathode active material are not suitable for use in HEVs requiring high output because HEVs demand high output power from the batteries, particularly when starting from a standstill, rapidly accelerating, and the like.
Meanwhile, crystalline graphite is mainly used as an anode active material, which has a very low discharge potential of about −3 V with respect to a standard hydrogen electrode potential, and exhibits very reversible charge/discharge behavior due to uniaxial orientation of a graphene layer and thus has excellent cycle lifespan.
However, such crystalline graphite has poor output properties and thus a secondary battery including such anode active material is not suitable for use as an energy source for HEVs requiring high output.